This invention relates to a digital attenuator which is inserted into a circuit on the receiving side of a digital echo suppressor so that attenuation nearly in proportion to the input PCM signal level may be attained.
In order to improve the echo attenuation effect, in conventional analogue echo suppressors, an analogue attenuator having the attenuation quantity of about 6 dB is inserted into a receiving circuit so that echo may be attenuated by an amount corresponding to the attenuation attained by this attenuator when the transmitting signal is higher than the receiving signal. In order to improve further the echo suppression effect, the analogue attenuator of the type described above is combined with the analogue amplitude compressor so that the echo attenuation quantity may be nearly proportional to the input audio signal level.
Upon the introduction of PCM communications systems, there have been proposed digital echo suppressors capable of suppressing echo without modifying or converting the PCM signal to an analogue signal. In general conventional digital echo suppressors used the digital attenuator in order to attenuate the PCM signal companded by about 6 dB by the piecewise linear compander as with the case of the conventional analogue attenuator.
However, conventional digital attenuators have the defects that the circuit arrangement is very complex because the complex logic operations must be carried out and that the attenuation quantity is constant without the input audio signal level. Therefore in order to attain an attenuation nearly in proportion to the input audio level, the conventional digital attenuator must be used in combination with the digital compressor whose function is similar to that of the analogue compressor. As a result, the circuit arrangement becomes more and more complex.